A liquid crystal display system comprises a liquid crystal display (LAD) and a back light or front light for illuminating the same. A transmission type LAD provided with a back light is set up such that light rays from the illumination unit are directed to the back surface of the LAD. Upon the light rays passing through the LAD, a light-and-dark pattern is created depending on changes in the light transmittance of the LAD, showing information in the form of textural or pictorial information.
The back light is generally broken down into the sidelight type (also called the edge type) and the underlying type depending on where the light source is located with respect to the display screen.
The sidelight type illumination unit generally comprises a linear light source such as a cold cathode fluorescent discharge tube and a light guide plate. This illumination unit is designed such that light rays leaving the linear light source are entered into the light guide plate from at least one side surface thereof (i.e., the entrance side surface), and then reflected at the back surface (i.e., the reflecting surface) of light reflectivity, leaving the front surface (i.e., the exit surface) of the unit.
Comprising a light source and a light diffusion plate, the underlying type illumination unit is designed such that light rays leaving the light source are entered into the light diffusion plate from the back (entrance) surface thereof, and then irregularly reflected by a light diffusion substance dispersed therein, leaving the front (exit) surface of the unit.
In recent years, light guide plates and light diffusion plates formed of lightweight, transparent resins having improved moldability (formidability) have been used. Mostly in the illumination unit for liquid crystal display systems, on the other hand, a fluorescent discharge tube (that is, a fluorescent lamp such as a hot or cold cathode fluorescent discharge tube) is used as a light source. The fluorescent discharge tube gives out ultraviolet rays together with visible light rays; as the light guide plate or light diffusion plate formed of a transparent resin is used over an extended period of time, they are susceptible to coloration due to the degradation of the transparent resin by ultraviolet rays. Consequently, while the liquid crystal display system is used over an extended period of time, the light leaving the illumination unit is colored (with a chromaticity increase), causing the quality of displays to become worse.
For an illumination unit having a light guide plate or a light diffusion plate, each formed of an acrylic resin, an approach has been proposed, wherein ultraviolet rays are cut out of light rays leaving a light source by varying the composition, etc. Of a glass tube that forms a fluorescent discharge tube. However, the ultraviolet rays that can be cut off by this approach are limited to those in a relatively short wavelength range (near 250-nm wavelength). For this reason, the approach is somewhat effective for acrylic resins having relatively higher resistance to degradation by ultraviolet rays among transparent resins; however, it is considerably less effective for other useful transparent resins. This still renders it difficult to develop and practically use an illumination unit for liquid crystal display systems, in which various transparent resins excellent in transparency and heat resistance are used for the light guide plate and light diffusion plate.